Argon recovery from ammonia plant purge gas utilizing a combination of cryogenic and non-cryogenic separating means

ABSTRACT

An improved high yield process is disclosed for argon recovery from an ammonia synthesis plant purge gas comprising hydrogen, nitrogen, argon, ammonia, and methane. In one embodiment of the present invention, this purge gas is subjected to the following steps: 
     (i) Separation of ammonia at high pressure by adsorption using zeolite molecular sieve material, which is subsequently regenerated by hot purge combined with pressure reduction; 
     (ii) Separation of methane and most of the nitrogen by pressure swing adsorption using a molecular sieve or an activated carbon material having greater selectivity for methane than argon; 
     (iii) Separation of hydrogen for recycle to the ammonia synthesis plant using a high pressure cryogenic distillation column or a membrane separator; and 
     (iv) Separation of the nitrogen by cryogenic distillation means to obtain essentially pure liquid argon product.

This application is a continuation-in-part of U.S. Ser. No. 06/832,206filed on Feb. 24, 1986 now U.S. Pat. No. 4,689,062.

BACKGROUND OF THE INVENTION

Economic production of argon via air separation plants is linked to theproduction of equivalent quantities of nitrogen or oxygen or both. Inrecent years, however, the demand for argon has been growing at a morerapid rate than the corresponding growth rate of either nitrogen oroxygen. Alternative sources for argon production have thus becomeattractive. One such alternative source is the purge gas from ammoniasynthesis plants.

In an ammonia synthesis plant, it becomes necessary to purge a fractionof the gas stream in order to maintain the inert concentration below aspecified level. Higher inert levels reduce the partial pressure of thereactants and cause an unfavorable shift of the ammonia synthesisreaction equilibrium. Methane and argon constitute the inert gases ofconcern. A typical composition of the ammonia purge gas available atapproximately 1900 psig pressure is as follows: 60.5% H₂, 20% N₂, 4.5%Ar, 13% CH₄ and 2% NH₃. Depending on the ammonia plant design, the purgegas may be available at much higher pressures or at slightly differentcompositions.

Present technology for argon recovery from ammonia plant purge gasemploys a cryogenic process that consists of a pre-treatment section forammonia removal and three cryogenic distillation columns. The first twocolumns are for stripping hydrogen and nitrogen in the feed gas and thefinal column is for separating argon and methane to obtain pure liquidargon product and also pure methane for use as fuel.

The primary object of the invention was to develop an improved processfor recovering argon from ammonia synthesis plant purge gas. A furtherobject of the present invention was to develop a process employing anadvantageous combination of non-cryogenic and cryogenic steps for argonrecovery from an ammonia synthesis plant purge gas. Yet a further objectof the present invention was to utilize a PSA system to accomplishremoval of methane from the purge gas exiting an ammonia synthesisplant.

In the following description of the invention, the term "pressure swingadsorption" or its acronym "PSA" is used in reference to a type ofprocess and apparatus that is now well known and widely used withrespect to separating the components of a gaseous mixture. A PSA systembasically comprises passing a feed gas mixture through one or moreadsorption beds containing a sieve material which has greaterselectivity for more strongly adsorbed components than more weaklyadsorbed components of the gas mixture. In the normal operation of atypical 2-bed PSA system, the connecting conduits, valves, timers, andthe like are coordinated and arranged so that when adsorption isoccurring in a first bed, regeneration is occurring in a second bed. Inthe usual cycle, sequential steps with respect to each adsorption bedinclude bed pressurization, product release and bed regeneration. BasicPSA systems are described in U.S. Pat. Nos. 2,944,627, 3,801,513, and3,960,522.

Various modifications and improvements to the basic PSA process andapparatus have been described in the literature, for example, in U.S.Pat. No. 4,415,340, issued on Nov. 15, 1983 and U.S. Pat. No. 4,340,398issued on July 20, 1982. The present invention is not limited to the useof any particular PSA process or apparatus design. A design that resultsin high argon yield, however, is detailed below as an example.

A new and improved process has been developed for recovering argon fromthe purge gas flowing from an ammonia synthesis plant. This processemploys a non-cryogenic means comprised of a pressure swing adsorption(PSA) unit for accomplishing a critical separation between argon andmethane as well as removing most of the nitrogen.

The present invention has several important advantages over the threestage prior art cryogenic recovery of argon. A considerable reduction incapital cost and operating expense is achieved through the use of a gasphase methane separation. In fact, the high pressure of the purge gasexiting from the ammonia plant can be used to provide most or all of theenergy requirements in the non-cryogenic separation. Furthermore, it ispossible, as a further energy saving measure, to pass the purge gasstream through a turbine in order to provide cooling needed for thelater cryogenic separation. Further advantages of the present processstem from the use of PSA units for ammonia separation and for methaneseparation ahead of a membrane for hydrogen separation. The ammoniapurge stream is at a cold temperature (about -10° F.), at which pressureswing adsorption is more effective. Because of the heats of adsorption,the product gas from the PSA units will be warmer. Membranes operatemore effectively at higher temperatures (about 70° F.). The temperaturesnoted here are for an ammonia plant operating at about 2000 psia and maybe slightly different in the event the plant is designed to operate atother pressures.

Another advantage of the present invention is that it offers the optionof separating ammonia simultaneously with methane and nitrogen in asingle PSA system. This option is advantageous when ammonia in the feedis at such low concentrations that the recovery or recycle of ammonia inthe purge gas is not critical. Expensive ammonia separation equipmentare then eliminated, making the process even more cost effective.Finally, the compact units employed in the present process are moreportable and, as a result, the purge gas available at numerous ammoniaplant sites over a wide geographical range can be more expeditiouslytapped for argon in order to meet the growing demand for this industrialgas.

BRIEF DESCRIPTION OF THE INVENTION

A first embodiment of the process of the present invention involves thefollowing stages:

(1) Ammonia from ammonia synthesis plant purge gas is removed bycontrolled adsorption at high pressures, on the order of 1100 psig,using zeolite molecular sieve material. Regeneration of this material iscarried out by purge with a hydrogen rich stream produced in the overallprocess. The hydrogen rich stream is optionally heated. Purge iscombined with pressure reduction to approximately 400 psig. The removedammonia may be recycled to the ammonia synthesis plant.

(2) The product gas, following ammonia adsorption, is passed to a PSAunit where essentially all of the methane and most of the nitrogen areremoved by using an adsorbent material possessing the necessaryselectivity.

(3) The product gas from the PSA system is passed to a cryogenicdistillation unit consisting of two cryogenic distillation columns. Inthe first column, hydrogen is separated; most of the hydrogen richstream is directly recycled, although a portion thereof may be used asthe purge gas for regeneration in the ammonia adsorption step and thenrecycled. In the second column, pure liquid argon is produced as abottom product, whereas a nitrogen product with small amounts ofhydrogen is the distillate. The argon usually lost in the vent gas ofthe PSA unit is substantially recovered in accordance with thisinvention by: recycling the vent gas (1) to the ammonia plant secondaryreformer; (2) to the PSA unit with the feed; or (3) recycling acocurrent depressurization product of the PSA unit to the PSA unit feed.

In an alternative embodiment, the product gas from the PSA unit is sentto a membrane separator for hydrogen removal and recycle. Part of thehydrogen stream may be used for regeneration in the ammonia adsorptionstep and then recycled. The non-permeate gas consisting of nitrogen andargon is then cryogenically treated in a single column to produce pureliquid argon.

Although ammonia adsorption is preferred, a conventional method forammonia removal may be employed, in which the purge gas is scrubbed withwater and the ammonia-water mixture is then separated by fractionation.Residual moisture in the gas leaving the scrubber can be removed byadsorption if the downstream processing cannot tolerate water content.

If ammonia in the feed is in small amounts and it is not critical torecover or recycle the same, then the ammonia adsorption of step (1) maybe combined in a single PSA unit with the methane and nitrogenseparation of step (2).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood by reference to thefollowing description of exemplary embodiments thereof in conjunctionwith the following drawings in which:

FIG. 1 discloses a schematic flow diagram, according to one embodimentof the present invention, of a process for recovering argon from anammonia synthesis plant purge gas; and

FIG. 2. discloses a schematic flow diagram according to a secondembodiment of the present invention;

FIG. 3 discloses a schematic of a PSA unit illustrating valve positionsand auxillary equipment according to one specific design; and

FIG. 4 is a timing diagram illustrating a full cycle sequence of PSAoperation.

FIG. 5 is a schematic process flow diagam of a process according to thepresent invention employing a first recycle option: PSA vent gas to theammonia plant secondary reformer.

FIG. 6 is a more detailed portion of the process diagram of FIG. 5,showing flows to and from the PSA unit.

FIG. 7 is a schematic of one possible configuration of the PSA unit,illustrating valve positions and auxillary equipment, as employed in thefirst recycle option.

FIG. 8 is a schematic process flow diagram of a portion of a processaccording to the present invention employing a second recycle option:vent gas recycle to the PSA unit.

FIG. 9 is a timing diagram illustrating a full cycle sequence of PSAoperation corresponding to the configuration shown in FIG. 7 andemploying the second recycle option.

FIG. 10 is a schematic process flow diagram of a portion of a processaccording to the present invention employing a third recycle option:recycle of cocurrent depressurization product to the PSA unit.

FIG. 11 is a schematic of one possible configuration of the PSA unit,illustrating valve positions and auxillary equipment, as employed in thethird recycle option.

FIG. 12 is a timing diagram illustrating a full cycle sequence of PSAoperation corresponding to the configuration shown in FIG. 11 andemploying the third recycle option.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, ammonia synthesis plant purge gas stream 1, at 1900psig and comprising approximately 60.5% hydrogen, 20% nitrogen, 13%methane, 4.5% argon, and 2% ammonia, enters a molecular sieve ammoniaadsorption unit 2 operating at an elevated pressure. The unit 2 isregenerated by purge with a gas consisting of optionally heated hydrogencoupled with pressure reduction to 400 psig. The purge gas is part ofthe distillate stream exiting the cryogenic column 9. The ammoniaremoved by unit 2 exits as stream 3 along with the gas used to purge theunit, and is recycled to the ammonia synthesis loop at 400 psig.

The product stream 5, following removal of ammonia in adsorption unit 2,and optional cooling by expansion means 15, such as a turbine, enters apressure swing adsorption (PSA) unit 6 for removal of essentially all ofthe methane and most of the nitrogen. The PSA unit 6 contains amolecular sieve adsorbent material which has a greater selectivity formethane than argon. The PSA vent stream 7 containing methane andnitrogen is usable as fuel for the ammonia plant. The product stream 8,comprising the predominant amount of argon, next enters a high pressurecolumn 9, where cryogenic distillation, at a typical pressure of 400psig, produces a top stream 10 of hydrogen containing a small amount ofnitrogen, which may be recycled to the ammonia synthesis plant afterbeing partly used as purge stream 4 for regeneration of unit 2. Thebottoms product stream 11, containing the concentrated argon, nextenters a second cryogenic distillation column 12, wherein remainingquantities of nitrogen and hydrogen exit the top of column 12 as gasstream 13. This may either be vented as a waste gas stream, used topurge the methane PSA unit 6, or used as fuel. The final product,essentially pure liquified argon, exits as stream 14 from the bottom ofcolumn 12. Argon yields of about 70 percent are obtainable at productmethane concentrations of under about 20 ppm. High purity argon productcontaining less than 0.5 ppm methane can also be achieved.

In the first embodiment of the present invention, described above inreference to FIG. 1, the pressure swing adsorption (PSA) unit 2 can bemade more effective by coupling with pressure reduction down to 400psig, which is the operating pressure of the high pressure hydrogenseparation column 9. Whereas the methane PSA unit operates atapproximately 400 psig or above for the cryogenic hydrogen separationembodiment, much higher pressures, for example, approximately 1100 psig,are required for the embodiment shown in FIG. 2.

Referring now to FIG. 2, an alternative embodiment is shown in which thetwo column cryogenic separation described above, is modified to a singlecolumn separation by use of a hydrogen membrane separator.

In FIG. 2, numbered items 1 to 8 are the same as described above inreference to FIG. 1. Upon exiting from the PSA unit 6, however, theproduct stream 8 enters a hydrogen membrane separator 9, which removeshydrogen as permeate stream 10. A small part of the permeate is used aspurge gas for regeneration of the ammonia adsorbing material. Thenon-permeate product stream 11 from the membrane separator 9, afterbeing cooled by expansion means 16, next enters a cryogenic distillationcolumn 12 in which an essentially pure liquid argon final product isobtained as bottoms stream 14. The more volatile nitrogen and hydrogencomponents of the product stream 11 exit from the top of the cryogenicdistillation column 12, to form stream 13 which may be vented, used aspurge gas for regeneration of the methane PSA unit 6, or used as fuel.

The ammonia adsorption unit 2 in FIG. 1 and FIG. 2, is suitably a threebed adsorber system described by Ruhemann (Petrocarbon Ltd.) in thearticle entitled "New developments in the treatment of ammonia purgegas", Indian J. Cryogenics 1982, Vol. 7, No. 3, pp 111-116. The threebeds operate at very high pressures (typically 1100 psig) and may beregenerated with a hot purge gas comprised of a hydrogen-nitrogenmixture generated later in the overall process. The regenerated ammoniaalong with the purge gas is recycled to the ammonia synthesis loop.

Alternatively, it is also possible that the ammonia and methaneadsorbers may be combined, in a PSA unit, provided the ammonia is not ofmuch value. For low ammonia concentrations in the feed, this is oftenthe case. In this case, regeneration is carried out at 25 psia or lowercombined with purge. The vent stream may be used as a low BTU fuel.

The adsorbent material for the PSA unit 6 shown in FIG. 1 and FIG. 2must have a greater selectivity for methane than argon. Both calcium andsodium aluminosilicate zeolites and activated carbons are suitablematerials. Carbon molecular sieves and silica molecular sieves are alsofunctional. Suitable zeolites include, but are not limited to thefollowing: 5A, 10X, 13X or mordenites. Preferred zeolites are 5A medicalgrade zeolite sold by Union Carbide, the 5A HC sieve sold by LaPorteIndustries or molecular sieves with comparable pore size and molecularattraction. The 5A medical grade zeolite was found to provide excellentargon-methane selectivity and to exhibit the ability to remove methanecompletely so that the PSA product gas will contain as low as fractionalppm levels. Removal of the methane to low levels is an importantcriterion; methane in the product gas would otherwise concentrate in thepure argon produced in the cryogenic section. Hence, expensivedownstream purification steps would be required if the PSA product gascontains undesirable levels of methane. A methane level of less than 20ppm is required and a level of less than 1 ppm is desired. A methanelevel of 0.5 ppm or below is preferred.

A suitable operating pressure in the PSA for methane separation is in arange of 50 psig to 1100 psig. A range of 400 to 1100 psig is preferredin order to maintain the pressure of the gases down stream and to avoiddownstream recompression. A pressure of 400 to 600 psi is preferred forthe first embodiment, whereas approximately 1100 psi is preferred forthe second embodiment.

A typical PSA unit is shown in FIG. 3. In FIG. 3, items 1 to 16represent conventional valves. Other items in the FIG. 3 are adsorptionbed A, adsorption bed B, equalization tank C, backfill tank D, productreservoir E and backpressure valve 17. This PSA unit may be operatedaccording to the full cycle sequence shown in Table I for a 2-bed PSAsystem with tank equalization, to reduce void gas loss, and with productbackfill, to achieve desired purities. In FIG. 4, the timing sequenceand valve position for each step of the sequence is shown.

                  TABLE I                                                         ______________________________________                                        Step                                                                          No.  Bed A          Bed B         Valves Open                                 ______________________________________                                        1    Bed Balance    Bed Balance   3,4,9,10,13                                 2    Feed Pressurization                                                                          Equalization  1,8,13                                                          with Tank                                                 3    Feed Pressurization                                                                          Vent          1,6,16,13                                   4    Constant Feed  Vent & Purge  1,6,15,16,13                                     & Product                                                                5    Constant Feed  Equalization  1,8,13                                           & Product      with Tank                                                 6    Constant Feed  Product Backfill                                                                            1,12                                             & Product                                                                7    Bed Balance    Bed Balance   3,4,9,10,13                                 8    Equalization   Feed Pressurization                                                                         2,7,13                                           with Tank                                                                9    Vent           Feed Pressurization                                                                         2,5,16,13                                   10   Vent & Purge   Constant Feed &                                                                             2,5,14,16,13                                                    Product                                                   11   Equalization   Constant Feed &                                                                             2,7,13                                           with Tank      Product                                                   12   Product Backfill                                                                             Constant Feed &                                                                             2,11                                                            Product                                                   ______________________________________                                    

By varying the product to feed ratio either by changing product flow orcycle time, the argon yield at various operating pressures thatcorresponded to zero methane concentration in the product can bedetermined by thermal conductivity analysis of the PSA product stream ina gas chromatograph. There is a moderate reduction in argon yield withincreasing pressure. The variation of argon yield with pressureindicates that the separation of methane is regeneration controlled; thehigher the amount of methane regenerated, the better will be theargon-methane selectivity.

The PSA unit must be regenerated periodically. Typical modes ofregeneration include (i) regeneration at tmospheric or slightly elevatedpressures coupled with product purge, (ii) regeneration at atmosphericor slightly elevated pressures coupled with purge using hydrogen or ahydrogen-nitrogen mixture, and (iii) vacuum regeneration.

When using product purge, it is preferred to restrict the purge todifferent portions of the half cycle. Typically, the product releasedimmediately after pressurization of the bed contains mostly hydrogen.The product released at the tail end of the production half cycle hasminimum purity. To take account of this fact, two purge steps may beemployed: the first purge step, immediately following pressurization ofthe adsorbing bed, and the second purge step towards the end of the halfcycle. By suitable choice of time for the two purge steps, the maximumargon yield with this mode of regeneration can be obtained.

A disadvantage of product purge is low argon yield due to loss of theproduct purge gas itself. For separating all of the methane in the feedgas, the purge gas requirement is considerable and accounts for a largepercentage of the argon lost.

An alternative to purge with product is purge with a gas available froma source external to the PSA; the hydrogen rich stream or thehydrogen-nitrogen mixture which are the distillate products from the twodownstream columns can be effectively used for purge.

A third alternative mode of regeneration is vacuum regeneration. Theyield obtained using vacuum regeneration is generally superior to theyield using purge. Vacuum regeneration, however, increases the capitalinvestment for the process slightly and increases the energy requirementappreciably. Since the vent stream is used as a fuel, recompression toabout 25 psia is also necessary unless special low pressure burners areinstalled. In determining the best regeneration procedure, the increasein argon yield that results with vacuum regeneration must be weighedagainst the incremental capital cost and energy requirements.

The operating pressures for the ammonia adsorber and the methane PSAunit are selected based on a typical ammonia plant design where thepurge gas is available at about 2000 psia and the hydrogen rich recyclegas is required to be at 350 psia for introduction to the second stageammonia compressors of the ammonia plant. It must be recognized that theprocess detailed here is applicable to any ammonia plant design; asuitable choice will have to be made for the pressures at variousseparation stages so that the recycle can be sent to the ammonia plantat a desired pressure.

The following working example illustrates a design based on actual plantdata, experimental results, or, where appropriate, theoreticalcalculations assuming well mixed streams.

EXAMPLE 1

An ammonia purge gas stream comprising approximately 60.5% hydrogen, 20%nitrogen, 13% methane, 4.5% argon, and 2% ammonia enters a three bedammonia adsorption unit containing 4 A molecular sieve material andoperating at 1200 psig. This adsorption unit removes the ammonia and isperiodically regenerated at 400 psig with a hydrogen rich purge stream.Ammonia rich vent is recycled to the synthesis compressor of the ammoniaplant. After being cooled by expansion, the ammonia depleted stream,comprising 61.7% hydrogen, 20.5% nitrogen, 4.5% argon, and 13.3%methane, enters a PSA unit comprising two beds containing 5 A medicalgrade molecular sieve material. The operating pressure within the bedsis approximately 450 psig during adsorption. This PSA unit removesessentially all of the methane and most of the nitrogen. The vent streamcomprises 9.0% hydrogen, 48.1% nitrogen, 4.0% argon and 38.9% methane.The product stream, comprising 88.9% hydrogen, 4.9% argon, and 6.2%nitrogen, enters a cryogenic distillation column operating at 400 psig.This cryogenic distillation column produces a top product of hydrogen,capable of being recycled to the ammonia synthesis plant, and a bottomsproduct stream of concentrated argon. This product stream is cooled byexpansion and next enters, at an operating pressure of about 30 psig, asecond cryogenic distillation column which, after removing residualnitrogen and hydrogen as a distillate, produces a liquified argon finalproduct. An argon yield of 68% with a methane concentration of 0.5 ppmis obtained.

Temperature, pressure, flow rate and composition of various streams aresummarized in Table II.

                                      TABLE II                                    __________________________________________________________________________    Stream                                                                              Temp. Pressure                                                                            Flow Rate                                                                           Composition (mole percent)                             (in FIG. 1)                                                                        (K.)  (psia)                                                                              (units/min)                                                                         H.sub.2                                                                           Ar  N.sub.2                                                                           CH.sub.4                                  __________________________________________________________________________    1     250-293                                                                             1900  100.0 60.5                                                                              4.5 20  13                                        3     240-293                                                                             400-600                                                                             2.0   (NH.sub.3 + Purge Gas)                                                  (+ Purge)                                                   5     250-293                                                                              400-1200                                                                           98.0  61.6                                                                              4.6 20.5                                                                              13.3                                      7     250-288                                                                             Min 1.5                                                                             33.4   9.0                                                                              4.0 48.1                                                                              38.9                                                  Max 25                                                            8     298*  Min 400                                                                             64.6  88.9                                                                              4.9 6.2 --                                                    Max 1200                                                          10    87    400   57.2  98.8                                                                              0.1 1.1 --                                        11     108**                                                                              400   7.4   12.4                                                                              41.4                                                                              46.2                                                                              --                                        13    83    25-60 4.3   20.9                                                                              1.3 77.8                                                                              --                                        14    98    25-60 3.1   --  100.0                                                                             --  --                                        __________________________________________________________________________     *Precooled before entering column 9                                           **Cooled by expansion before entering column 12                          

In the PSA separation, methane and nitrogen are removed from the feed asvent gas. A certain amount of argon is lost with the vent gas. Theselosses arise because of two reasons: (i) argon is adsorbed in the sievesat the PSA operation pressure, and (ii) argon from the bed voids isdischarged during pressure reduction. While maintaining the purity inthe argon rich primary product, the once-through argon yield istypically about 70 percent. Certain process modifications which recycleall or part of the PSA vent gas result in an overall argon yield of 90percent or better. Modifications to increase yield include thefollowing: (i) vent gas recycle to the ammonia plant secondary reformer,(ii) vent gas recycle to the PSA unit, and (iii) recycle of cocurrentdepressurization product to the PSA unit.

First Recycle Option: Vent gas recycle to the ammonia plant secondaryreformer involves pressurizing all or part of the PSA vent gas (250 to525 psig depending on the specific ammonia plant design) and recyclingit to the ammonia plant secondary reformer. A simplified flow sheet ofthe ammonia plant with this recycle option is shown in FIG. 5. It ispossible to employ a total recycle option or a partial recycle option.In the total recycle option, all of the vent gas from the PSA bedsemployed to separate methane and nitrogen is recycled to the ammoniaplant secondary reformer. The recycle stream will be about 2 to 3percent by volume of the total dry gas entering the secondary reformer.The recycle stream provides methane for conversion to hydrogen in thesecondary reformer as well as hydrogen and nitrogen. This will result inadditional ammonia production which is of value to the ammonia plant. Ifit is desired to maintain the ammonia production level, the recyclestream will lead to fuel cost savings by cutting back on the natural gasfed to the process. If the purge gas rates are kept constant after therecycle, the argon concentration in the purge will increase. If theargon concentration in the ammonia synthesis loop has to be maintainedat present levels, the purge flow has to be increased. In either case,changes to be incorporated in the ammonia plant are minimal because thevolume of recycle gas is very small compared with the main flow in thesecondary reformer. The total recycle option can completely eliminatePSA argon losses and result in a net argon yield greater than 90percent.

It is also possible to collect the PSA vent gas in two fractions, thusleading to a partial vent gas recycle option. The first fraction isobtained by reducing pressure in the PSA beds from the equalizedpressure to the regeneration pressure or somewhere in between the two.This fraction can contain from 50 to 80 percent of the total argon lostin the vent and is typically only 30 percent by volume of the totalvent. The secondary vent fraction containing the regeneration gas andthe desorbed gas is sent back as fuel gas to the ammonia plant. Thefirst fraction is compressed to secondary reformer operating pressureand recycled. With this partial recycle option, net argon yield can beas high as 90 percent. The compression cost is less than thecorresponding cost for the total recycle option. The recycle gas flow isless than one percent of the main flow into the secondary reformer tothe ammonia plant and hence this option requires very little change inpurge gas flow to maintain current inert levels.

Second Recycle Option: As noted above, the vent gas may be collected intwo fractions. In the second recycle option, vent gas recycle to the PSAunit, the first fraction containing a major portion of the lost argon iscompressed to PSA operating pressure and mixed with fresh feed to thePSA unit. The total feed to the PSA unit is increased and more methaneneeds to be processed as the methane removed as fuel gas in thesecondary vent fraction must equal the methane in the fresh feed. WithPSA recycle, overall argon yield can be increased to between 85 and 90percent. Associated cost factors include compression of the recycle gas,additional load to the PSA unit and a moderate increase in nitrogen tobe processed in the column.

In the partial recycle case, the pressure at which the first fraction iscollected serves as a variable to control and maintain the quantity ofgas to be recycled. There is a trade-off between the cost involved withrecycling the gas and the value of the argon that is recovered by netyield improvement.

Third Recycle Option: In this recycle option, cocurrent depressurizationproduct is recycled to the PSA unit. The PSA vent gas is normallycollected in a direction countercurrent to the feed gas. During theproduction cycle, concentration fronts are formed for each of thecomponents in the feed. Components that are strongly adsorbed (eg.methane) exist at feed concentration in the gas phase near the entranceof the bed. Over a length equal to the equilibrium saturation zone, thegas phase concentration is constant. Beyond this length theconcentration decreases sharply. In the present separation (methane andnitrogen from the feed gas), it becomes necessary to halt the productionof argon rich primary product when the equilibrium methane front is wellwithin the PSA bed since only a fractional ppm level methane in theprimary product can be tolerated. The product end of a PSA bed at thecompletion of the production step thus contains predominant amounts ofargon which mainly accounts for the argon losses with the vent. It hasbeen found to be desirable to collect a secondary product in a directioncocurrent to feed by moderate pressure reduction of the PSA beds, beforecountercurrent vent is initiated. The cocurrent product contains asignificant amount of argon along with nitrogen and only a small amountof methane. The quantity of gas cocurrently produced and the methaneconcentration can be controlled by regulating the pressure at which thisproduct is stopped. The total quantity of this product is only about 15percent of the vent gas. Hence recycle of the cocurrent product ratherthan partial recycle of countercurrent vent gas reduces the compressioncost and the incremental PSA cost. Typically, the cocurrent productcontains half of the argon normally lost with the vent gas and henceoverall argon yield can be increased to nearly 85 percent with thisoption.

EXAMPLE 2

This example illustrates a process modification employing partial ventgas recycle to the ammonia plant secondary reformer which isschematically shown in FIG. 6. The first fraction of the PSA vent gas iscollected in a surge tank, compressed to ammonia plant secondaryreformer pressure (250-400 psia) and recycled. This recycle gastypically, constitutes only 2% or less of the main dry gas volumetricflow into the secondary reformer. The vent gas recycle to the secondaryreformer enables the ammonia plant operator to cut the air flow to thecompressor as well as the fuel (CH₄) to the process. The cut back in airand fuel will correspond to the quantity of nitrogen and methaneequivalent in the recycle vent stream. The argon in the recycle streamconcentrates in the ammonia plant and is recycled as feed to the argonrecovery process, the feed thus becomes richer in argon. Typically, theargon concentration in the purge gas increases from 4.5 to 4.8 percent,whereas the total inert concentration (argon plus methane) increasesfrom 17.5 to 17.8. A PSA configuration for this process modification isshown in FIG. 7. Referring to FIG. 7, the PSA unit comprises adsorptionbeds A and B, equalization tank C, backfill tank D, product reservoir E,recycle vent surge tank F, recycle compressor G, back pressure regulator18 and solenoid valves 1 through 17. The accompanying full cyclesequence is given in the Table III below. The temperature and pressureconditions, flow rates and composition of various streams as referred inFIG. 6, are summarized in Table IV.

                  TABLE III                                                       ______________________________________                                        Cycle Sequence for PSA with Recycle                                           Step                                                                          no:  Bed A         Bed B         Valves Open                                  ______________________________________                                        1.   Bed balance   Bed balance   3, 4, 13, 14, 17                             2.   Equalization with                                                                           Product backfill                                                                            9, 16                                             tank                                                                     3.   Equalization with                                                                           Feed Pressurization                                                                         2, 9, 17                                          tank                                                                     4.   Vent to atmosphere                                                                          Constant feed &                                                                             2, 5, 17                                                        product release                                            5.   Vent and purge                                                                              Constant feed &                                                                             2, 7, 11, 17                                                    product release                                            6.   Equalization with                                                                           Constant feed &                                                                             2, 9, 17                                          tank          product release                                            7.   Bed balance   Bed balance   3, 4, 13, 14, 17                             8.   Product backfill                                                                            Equalization with                                                                           10, 15                                                          tank                                                       9.   Feed pressurization                                                                         Equalization with                                                                           1, 10, 17                                                       tank                                                       10.  Constant feed &                                                                             Vent to atmosphere                                                                          1, 6, 17                                          product release                                                          11.  Constant feed &                                                                             Vent and purge                                                                              1, 8, 12, 17                                      product release                                                          12.  Constant feed &                                                                             Equalization with                                                                           1, 10, 17                                                       tank                                                       ______________________________________                                    

                                      TABLE IV                                    __________________________________________________________________________    Stream Conditions - Vent Gas Recycle to Ammonia Plant Secondary Reformer      Stream                                                                              Temp Pressure                                                                            Flow Rate                                                                           Composition (volume percent)                           (in FIG. 6)                                                                         (K.) (psia)                                                                              (units/min)                                                                         H.sub.2                                                                           Ar  N.sub.2                                                                           CH.sub.4                                   __________________________________________________________________________    1     298.0                                                                              425.0 125.4 61.5                                                                              4.9 20.3                                                                              13.3                                       2     288.0                                                                              Min 15                                                                              23.8  8.9 6.2 42.8                                                                              42.1                                                  Max 100                                                            3     303.0                                                                              250-400                                                                             23.8  8.9 6.2 42.8                                                                              42.1                                       4     298.0                                                                              410.0 82.7  88.6                                                                              5.2 6.2 --                                         5     288.0                                                                              Min 1.5                                                                             24.2  10.9                                                                              1.8 59.8                                                                              27.5                                                  Max 25                                                             6     108.0                                                                              400   9.4   9.7 44.3                                                                              46.0                                                                              --                                         7      87.0                                                                              400   73.3  98.7                                                                              0.1 1.2 --                                         8      83.0                                                                              25-60 4.1   --  100.0                                                                             --  --                                         9      98.0                                                                              25-60 5.3   17.2                                                                              1.2 81.6                                                                              --                                         __________________________________________________________________________

EXAMPLE 3

This example illustrates vent gas recycle for the PSA portion of theprocess, schematically shown in FIG. 8. A purge feed gas stream afterammonia removal, available at 425 psia, comprising 61.6 H₂, 20.5 N₂, 4.6Ar and 13.3 percent CH₄ is mixed with compressed recycle vent gas andfed at ambient temperature to the PSA unit, comprising beds containing5A Medical grade aluminosilicate zeolite. A typical PSA configurationdepicting the various valves is the same as shown in FIG. 7 (in thiscase, however, the outlet of the vent gas recycle compressor isconnected to the feed line). The PSA unit is operated in accordance withthe full cycle sequence as shown in Table III above. The PSA vent gas iscollected in two fractions; the first fraction merely by beddepressurization, while the second fraction by purge gas desorptionalong with the purge gas. The second fraction is used as fuel in theprocess, whereas the first fraction is collected in a surge tank,compressed to PSA operating pressure and mixed with the fresh feed tothe process. The temperature and pressure conditions, flow rates andcomposition of various streams in FIG. 8, are summarized in Table V.FIG. 9 is a timing diagram of a process employing vent gas recycle tothe PSA feed.

                                      TABLE V                                     __________________________________________________________________________    Stream Conditions - Vent Gas Recycle to PSA Feed                              Stream                                                                              Temp Pressure                                                                            Flow Rate                                                                           Composition (volume percent)                           (in FIG. 8)                                                                         (K.) (psia)                                                                              (units/min)                                                                         H.sub.2                                                                           Ar  N.sub.2                                                                           CH.sub.4                                   __________________________________________________________________________    1     298  425   98.0  61.6                                                                              4.6 20.5                                                                              13.3                                       2     298  425   134.5 46.3                                                                              4.4 24.9                                                                              24.2                                       3     288  425   36.5  4.6 5.3 36.7                                                                              53.4                                       4     303  410   71.2  84.8                                                                              5.8 9.4 --                                         5     288  Min 1.5                                                                             33.3  3.0 1.1 56.7                                                                              39.2                                                  Max 25                                                             6     108.0                                                                              400   10.5  7.5 38.3                                                                              54.2                                                                              --                                         7     87.0 400   60.7  98.1                                                                              0.2 1.7 --                                         8     83.0 25-60 4.0   --  100.0                                                                             --  --                                         9     98.0 25-60 6.5   12.1                                                                              0.6 87.3                                                                              --                                         __________________________________________________________________________

EXAMPLE 4

This example illustrates cocurrent product recycle, schematically shownin FIG. 10 for the PSA portion of the process. A purge gas feedstreamafter ammonia removal, comprising 61.6 H₂, 20.5 N₂, 46 Ar and 13.3 CH₄,at a pressure of 425 psia, is mixed with the cocurrent product recyclestream and fed at ambient temperature to the PSA unit, comprising bedscontaining 5A Medical grade aluminosilicate zeolite. A typical PSAconfiguration depicting the various valves is shown in FIG. 11.Referring to FIG. 11, the PSA unit comprises adsorption bed A,adsorption bed B, equalization tank C, backfill tank D, productreservoir E, cocurrent product surge tank F, recycle compressor G, backpressure regulator 18 and solenoid valves 1 through 17.

The beds A and B are physically divided into two vessels 1A, 2A and 1B,2B to facilitate removal of the cocurrent product stream from anintermediate position in the bed. The cocurrent product is drawn at anintermediate pressure (for example, 45 psia) and compressed to PSAoperating pressure before being mixed with fresh feed. The PSA unit isoperated in accordance with the full cycle sequence shown in Table VIbelow. FIG. 12 shows a timing diagram for the full cycle sequence. Thetemperature and pressure conditions, flowrates and composition ofvarious streams in FIG. 10, are summarized in Table VII.

                  TABLE VI                                                        ______________________________________                                        Cycle Sequence for PSA with Cocurrent Product Recycle                         Bed A          Bed B         Valves Open                                      ______________________________________                                        1.   Bed balance   Bed balance   3, 4, 11, 12, 17                             2.   Equalization with                                                                           Product backfill                                                                            9, 16                                             tank                                                                     3.   Equalization with                                                                           Feed Pressurization                                                                         2, 9, 17                                          tank                                                                     4.   Cocurrent     Constant feed &                                                                             2, 7, 17                                          production    product release                                            5.   Vent to atmosphere                                                                          Constant feed &                                                                             2, 5, 17                                                        product release                                            6.   Vent and purge                                                                              Constant feed &                                                                             2, 5, 13, 17                                                    product release                                            7.   Equalization with                                                                           Constant feed &                                                                             2, 9, 17                                          tank          product release                                            8.   Bed balance   Bed balance   3, 4, 11, 12, 17                             9.   Product backfill                                                                            Equalization with                                                                           10, 15                                                          tank                                                       10.  Feed pressurization                                                                         Equalization with                                                                           1, 10, 17                                                       tank                                                       11.  Constant feed &                                                                             Cocurrent     1, 8, 17                                          product release                                                                             production                                                 12.  Constant feed &                                                                             Vent to atmosphere                                                                          1, 6, 17                                          product release                                                          13.  Constant feed &                                                                             Vent and purge                                                                              1, 6, 14, 17                                      product release                                                          14.  Constant feed &                                                                             Equalization with                                                                           1, 10, 17                                         product release                                                                             tank                                                       ______________________________________                                    

                                      TABLE VII                                   __________________________________________________________________________    Stream Conditions - Cocurrent Product Recycle Process                         Stream Temp Pressure                                                                            Flow Rate                                                                           Composition (volume percent)                          (in FIG. 10)                                                                         (K.) (psia)                                                                              (units/min)                                                                         H.sub.2                                                                           Ar  N.sub.2                                                                           CH.sub.4                                  __________________________________________________________________________    1      298.0                                                                              425   98.0  61.6                                                                              4.6 20.5                                                                              13.3                                      2      298.0                                                                              425   108.4 58.0                                                                              5.0 24.1                                                                              12.9                                      3      298.0                                                                              425   10.4  24.1                                                                              8.8 58.0                                                                               9.1                                      4      303.0                                                                              410   69.4  87.0                                                                              5.4  7.6                                                                              --                                        5      293.0                                                                              Min 1.5                                                                             32.6   9.2                                                                              2.3 48.5                                                                              40.0                                                  Max 25                                                            6      108.0                                                                              400   10.5  23.0                                                                              35.1                                                                              41.9                                                                              --                                        7       87.0                                                                              400   58.9  98.5                                                                              0.1  1.4                                                                              --                                        8       83.0                                                                              25-60 3.6   --  100.0                                                                             --  --                                        9       98.0                                                                              25-60 6.9   35.6                                                                              *   64.6                                                                              --                                        __________________________________________________________________________     *Trace quantity                                                          

We claim:
 1. A process for the recovery of argon from a gas mixturecomprising hydrogen, nitrogen, methane, argon, and ammonia, the stepscomprising:(i) Passing said gas mixture to a means for removing ammoniato produce an ammonia depleted gas mixture (ii) passing said ammoniadepleted gas mixture to a pressure swing adsorption means for removingessentially all of the methane and most of nitrogen thus producing a gasmixture depleted in methane; (iii) passing said gas mixture to a meansfor removing most of the hydrogen thus producing a product streamdepleted in hydrogen; and (v) passing said stream to a cryogenicdistillation column for the separation of remaining amounts of hydrogenand nitrogen to produce a high purity argon product; the process furthercomprising recycling all or part of the vent gas from the pressure swingadsorption means into the process or, in the event said gas mixture isthe purge gas from an ammonia synthesis plant, to said plant.
 2. Theprocess of claim 1, wherein said vent gas is recycled into the processto the pressure swing adsorption means via the feed stream thereto.
 3. Aprocess in accordance with claim 2, wherein the vent gas is collected intwo fractions, the first fraction is recycled to the pressure swingadsorption means and the second fraction is used for fuel in saidprocess.
 4. The process of claim 1, wherein the ammonia synthesis plantincludes a secondary reformer, and said vent gas is recycled to saidsecondary reformer.
 5. The process of claim 4, wherein the vent gas iscollected in two fractions, the first fraction is recycled to thesecondary reformer and the second fraction is used for fuel in saidplant.
 6. A process for the recovery of argon from a gas mixturecomprising hydrogen, nitrogen, methane, argon, and ammonia, the stepscomprising:(i) Passing said gas mixture to a means for removing ammoniato produce an ammonia depleted gas mixture; (ii) passing said ammoniadepleted gas mixture to a pressure swing adsorption means for removingessentially all of the methane and most of nitrogen thus producing a gasmixture depleted in methane; (iii) passing said gas mixture to a meansfor removing most of the hydrogen thus producing a product streamdepleted in hydrogen; and (iv) passing said stream to a cryogenicdistillation column for the separation of remaining amounts of hydrogenand nitrogen to product a high purity argon product; the process furthercomprising recycling into said process a cocurrent depressurizationproduct from the pressure swing adsorption means employed for methaneremoval in step (ii).
 7. The process of claim 6, wherein the cocurrentdepressurization product for recycle is obtained from the product end ofthe said pressure swing adsorption means.
 8. The process of claim 6,wherein the cocurrent depressurization product for recycle is obtainedfrom an intermediate location on the pressure swing adsorption means. 9.The process of claims 1 or 6, wherein said means for removing ammoniacomprises a zeolite adsorption bed.
 10. The process of claims 1 or 6,wherein said zeolite adsorption bed operates at a pressure in the rangeof 400 to 1900 psig.
 11. The process of claims 1 or 6, whereinregeneration of said zeolite adsorption bed is carried out by passing ahot mixture of nitrogen and hydrogen gas through said adsorption bed atthe same or reduced pressure.
 12. The process of claims 1 or 6, whereinsaid means for removing hydrogen is a membrane separator.
 13. Theprocess of claims 1 or 6, wherein said means for removing hydrogen is ahigh pressure cryogenic distillation column.
 14. The process of claims 1or 6, wherein the methane concentration in said argon product is equalto or less than 20 ppm.
 15. The process of claims 1 or 6, wherein themethane concentration in said argon product is equal to or less than 1ppm.
 16. The process of claims 1 or 6, wherein said argon product is aliquid.
 17. The process of claims 1 or 6, wherein said means forremoving essentially all of the methane and most of the nitrogencomprises at least one adsorption bed containing an adsorbent materialwhich exhibits a greater selectivity for methane than argon.
 18. Theprocess of claim 17, wherein said adsorption bed operates at a pressurein the range 400 to 1100 psig.
 19. The process of claim 17, wherein saidadsorbent material is an activated carbon.
 20. The process cf claim 17,wherein said adsorbent material is a molecular sieve.
 21. The process ofclaim 20, wherein said molecular sieve is an aluminosilicate zeolite.22. The process of claim 21, wherein said aluminosilicate is selectedfrom the group consisting of 5A, 10X, 13X or mordenites.
 23. The processof claim 22, wherein said aluminosilicate zeolite is 5A medical gradesieve or 5A HC sieve.